The invention relates to the production of cast cylinder heads made of aluminium alloys comprising at least two different alloys. The liquid alloys may comprise solid particles at the time of casting of varied size and shape so as to produce composites with a metal matrix after solidifying.
This technique makes it possible to optimise the choice of the materials according to the main functions required in the different parts of the cylinder heads. By way of illustration there may be mentioned the requirement of a maximum tolerance to damage by heat in the vicinity of the combustion chamber, especially in the regions between the valve seats. On the other hand, in the cold part of the cylinder head, especially the securing posts, the critical property is mechanical strength, so as to endow the cylinder head with maximum stiffness and the best possible aptitude to clamping, with a minimum weight of the finished component.
At present, however, there is no manufacturing technique permitting the problem specified above to be solved in a satisfactory and economically viable manner.
In fact, it is possible, to be sure, to investigate materials exhibiting both a high mechanical strength and good heat resistance. However, experience shows that materials of this type are costly. For example, according to the manufacturers' estimates, metal matrix composites reinforced with silicon carbide particles of the Duralcan type cost 2 to 3 times more than conventional casting alloys, and this rules out their use for the whole of the cylinder head.
In general, the use of high-performance materials must be restricted to a local application in the regions where they are indispensable, this being due to their cost.
Furthermore, so far as we are aware, there is no technique in existence enabling such materials to be inserted into a cylinder head. The insertion of aluminium alloys or of metal matrix composites (for example the AlFe AlFeCe alloys obtained by powder metallurgy, followed by bonding, high heat-performance alloys obtained by a process of the Osprey type, metal matrix composites resulting from the impregnation of preforms, for example by liquid forging--Squeeze Casting--etc) placed in the solid state in the cylinder head at the time of casting comes up against the difficulty of successful metallurgical bonding between the material of the cylinder head and that of the insert(s).
Finally, another route which is at present developed for locally reinforcing the material of a cylinder head consists of impregnation when preforms are being cast (especially with alumina or silicon carbide or reinforcements consisting of long fibres). However, technology of this type introduces high manufacturing overcosts when compared with the usual techniques of gravity and/or low pressure casting, especially because of the need to produce a partial vacuum and then to apply overpressures of several Pa which make it necessary to cover the sand cores with a protective film so that they themselves are not impregnated with liquid metal.